Glaucoma is a major cause of irreversible blindness in the United States. Approximately one percent of the general population has this disease. The disease is characterized by elevated intraocular pressure, optic nerve damage, and visual field loss. Symptoms of the disease may include eye pain and visual disturbances, but usually it is asymptomatic. There are many types of glaucoma, the most common type being primary openangle glaucoma. This type of glaucoma usually occurs in older people and may also have a hereditary predisposition.
Most glaucomas have elevated intraocular pressure as a major characteristic. It is therefore important to be able to measure this parameter accurately for both the diagnosis and the treatment of glaucoma. The intraocular pressure in normal people, however, varies throughout the day. It is usually highest in the early morning and lowest in the evening. The size of this fluctuation is believed to be accentuated in people with glaucoma. Therefore, an intraocular pressure measurement at a single point of time may not tell the whole story. A series of intraocular pressure readings taken at different times of the day and at night is more important in the assessment of a patient with glaucoma. A normal intraocular pressure reading in the physican's office does not rule out the possibility of a higher intraocular pressure "spike" occurring at another time at home or at work.
Intraocular pressure is usually measured with a tonometer. Clinical tonometers operate by measuring the force required to momentarily deform or depress an area on the surface of the eye and then relating this force to the intraocular pressure. These instruments are only capable of making an instantaneous or "spot" measurement of intraocular pressure. The desirability of making continuous intraocular pressure measurement, however, has been recognized and led to several efforts to design a suitable device.
One such instrument uses strain gauges mounted in soft contact lenses that sense the deformation of the meridional angle of juncture between the cornea and sclera to measure changes in intraocular pressure. These strain gauges have to be positioned exactly over the corneoscleral junction to obtain maximum output; and, the soft contact lenses have to be individually fit, molded, and calibrated for each subject's eye because of individual differences in the meridional angle of juncture.
Another instrument uses a miniature scleral applanating device that has a transensor consisting of a passive resonant coil/capacitor combination which is made pressure sensitive by the movement of a small ferrite plate which acts as its applanating surface. Oscillation induced in the transensor by a remote grid dip oscillator is monitored by a digital frequency counter. The resonant frequency of oscillation in the transensor is then linearly related to the in vitro intraocular pressure. This instrument, too, suffers from many disadvantages and drawbacks. Because of the effect on the resonant frequency of the ferrite plate, the accuracy of this instrument can vary according to temperature, atmospheric pressure, coupling of the transensor to the eye, physical properties of the sclera, mechanical instability of the transensor, permeability of the transensor to saline, and the geometric relationship between the transensor and the aerial system. The reproducibility of intraocular pressure readings between eyes over a period of time is poor. Ocular rigidity has a significant effect on the calibration curves. Calibration may be necessary for individual eyes and species.
Still another type of instrument in the prior art employs a suction cup designed to fit the periphery of the cornea and to applantate its central part. A slow, continuous saline infusion entering through a central opening forms a disc of fluid between applanating and applanated surface in which the pressure is followed by a conventional pressure transducer. The saline leaves the periphery of the cup via a hanging tube creating a suction pressure of approximately 15 mm. Hg, which keeps the cup on the cornea. This instrument is quite reproducible in its measurements, but it tends to overestimate the intraocular pressure. Also it is not very portable and the tested subject is not able to see during the pressure measurements.
In contrast with these various prior art devices, the ideal non-invasive, continuous intraocular pressure monitoring device should have the following features: (1) It must be accurate, reproducible, and independent of gravity in its measurements; (2) The tested subjects should be able to wear the device safely, comfortably, and conveniently without disturbance of vision or of rountine daily activities, including sleeping and taking any ocular medications; and (3) The device should also be simple to operate, independent of subjective judgment from the operator, and inexpensive to purchase and maintain. The instrument of this invention meets all of these important criteria.